This plugin demo shows how to make a main screen plugin.
The main screen appears as a button next to the "2D", "3D", "Script", and
"AssetLib" buttons. It also shows up when a Node it handles
is selected.
For more information, see this documentation article: https://docs.godotengine.org/en/latest/tutorials/plugins/editor/making_main_screen_plugins.html
If you would like to see a more complete example of what main screen plugins are capable of, check out the 2.5D demo project.